This invention relates to a method of making a bleaching clay effective for the removal of color impurities found in mineral, vegetable, and animal oils.
Acid treated clays are routinely used for adsorptive purification of petroleum products and oils (both edible and nonedible) derived from vegetable or animal sources. With the traditional focus having been on the decolorizing effect, clay and clay-like materials used for oil purification are commonly called bleaching clays. Targeted color impurities typically present in oils include, for example, carotenoids, xanthophylls, xanthophyll esters, chlorophyll, tocopherols, as well as oxidized fatty acids and fatty acid polymers.
A historical method of removing colored impurities from oils involved intimately contacting the oil with xe2x80x9cFuller""s earth,xe2x80x9d an imprecise label applied to mixtures of clay minerals having oil-bleaching properties, but thought to contain mostly palygorskite, sepiolite, and smectite. See A.D. Rich, xe2x80x9cBleaching Clay,xe2x80x9d Industrial Minerals and Rocks, 3rd Ed., AIMMPE, N.Y., pp. 93-101 (1960). In the early 1900s, it was discovered that the ability of Fuller""s earth and related clay minerals to adsorb or otherwise eliminate color impurities from oil is improved by acid activation. Accordingly, acid-activated clays are more widely used for bleaching purposes.
In effect, conventional acid treatment of clay replaces exchangeable cations, such as K+, Na+, and Ca2+, by H+ in the intralamellar space of the clay""s crystalline lattice and also leaches out some of the Al3+, Fe3+, and Mg2+ metal cations. Acid treatment thus renders the clay physically more porous and electrochemically more active.
The general process of acid-activation of clays, as used today in industry, includes grinding clay for uniform activation and dispersion in water, slurrying the ground clay in water and then adding acid to the clay slurry. Usually about 6 to 8 hours of acid digestion is necessary to properly acid-activate the clay. The digested slurry then is washed substantially free of dissolved salts, such as iron and aluminum sulfates or chlorides, depending upon the particular mineral acid used. The acid-activated clay then is dewatered, such as by filtration, and the resulting filter cake is dried and subsequently ground to product specifications.
The following patents describe various processes for treatment of clay to enhance the filtering characteristics of the clay in the removal of colorizing compounds: U.S. Pat. No. 1,402,112 to Tellier; U.S. Pat. No. 1,408,656 to Stratford; U.S. Pat. No. 1,408,655 to Stratford; U.S. Pat. No. 1,524,843 to Ruprecht; U.S. Pat. No. 1,544,210 to Bierce; U.S. Pat. No. 1,731,702 to Black; U.S. Pat. No. 1,739,734 to Raine et al.; U.S. Pat. No. 3,029,783 to Sawyer et al.; U.S. Pat. No. 3,617,215 to Sugahara; U.S. Pat. Nos. 4,717,699; 4,847,266; and 4,919,818 to Alexander.
U.S. Pat. No. 3,617,215 to Massaire et al., for example, describes a process for the production of acid-activated bleaching clay that entails mixing an aqueous clay slurry with a high dose of concentrated acid at elevated temperatures (100xc2x0 C. to 121xc2x0 C.), followed by washing and drying of the acidulated product. This prior art method results in high levels of residual acid effluent and salts thereof whose removal and disposal costs are expensive, however. Other commercial manufacturing process for the production of bleaching clay avoids the use of large amounts of concentrated acid but requires a high-temperature (about 1000xc2x0 F.), and therefore high-cost, calcining step to calcine the raw clay (e.g. U.S. Pat. No. 3,029,783 to Sawyer et al.).
The raw clay starting materials used to produce bleaching clays contain varying types of sand, which may include quartz, feldspar, mica and other relatively hard minerals. Because they foul oil processing equipment and offer at most negligible bleaching action, these sand constituents are, in effect, inert bleaching clay contaminants.
The present invention concerns an improved method of producing activated bleaching clay having a reduced amount of inert constituents.
A bleaching clay manufacturing method according to the present invention provides improved acid activation of the clay mineral without substantial deterioration of the clay mineral structure. The bleaching clay product is prepared by slurrying particulate palygorskite clay starting material with water to form an aqueous slurry, removing sand from the aqueous slurry to form a beneficiated slurry, conditioning the beneficiated slurry by combining therewith a conditioning agent such as a pH modifier, a Lewis acid salt, and mixtures thereof, separating the conditioned slurry into an acidic filtrate and a conditioned clay concentrate, and drying the conditioned clay to a free flowing particulate mass, usually having a free moisture content of about 5 to 25 percent by weight, preferably 10 to 20 percent by weight.
A preferred palygorskite clay starting material is a particulate naturally occurring palygorskitexe2x80x94smectite clay mineral having a pH value in the range of about 5 to about 9, preferably about 7 to about 9. The slurrying step is conducted such that the aqueous slurry has a clay solids content in the range of about 5 to about 30 weight percent, based on the total weight of the slurry. At a solids content above about 20 percent by weight a dispersant can be added, if desired, to enhance flow properties. In the slurry conditioning step, the beneficiated slurry is combined with a Lewis acid salt in an amount in the range of about 0.25 to about 3 weight percent (based on the weight of the solids) and/or a sufficient amount of a pH modifier to adjust the pH of the slurry to a value in the range of about 0.5 to about 6. The solid/liquid separation step is conducted such that the conditioned clay concentrate has a solids content of 15 up to 50 weight percent, preferably 40 to 50 weight percent, based on the total weight of the concentrate.
The resulting bleaching clay may be utilized for bleaching purposes in numerous applications for a wide variety of edible oils and the like.